Embodiments of the invention relate to ion exchange processes and apparatus.
Ion exchange cells are used to remove or replace dissolved solids or ions in solutions. For example, ion exchange membranes and beads are used to deionize water to produce high purity drinking water by removing contaminant and other dissolved solids from a municipal wastewater streams. Ion exchange is also used for the selective substitution of ions in the treatment of industrial wastewater. In another example, tap water is softened by replacing hard divalent ions in tap water, such as calcium, with soft monovalent ions, such as sodium or potassium. Typically, ion exchange efficiency is measured by determining the total dissolved solids (TDS) content of a treated and untreated solutions and reported as a percentage reduction (% R).
Electrolytic assisted ion exchange improves ion extraction efficiency and provides easier regeneration of the ion exchange material in the cell. In such a system, an electric field is applied across a water-splitting ion exchange membrane, as described in commonly assigned U.S. Pat. No. 5,788,826 to Nyberg, which is incorporated herein by reference in its entirety. The water splitting membrane typically comprises a strong-acid cation exchange surface or layer (sulfonate groups; —SO3M) and a strong-base anion exchange surface or layer (quaternary ammonium groups; —NR3A). The membrane is positioned between electrodes so that its cation exchange surface faces the first electrode and its anion exchange surface faces the second electrode. During a deionization process cycle, a solution stream is passed through the cell while a predetermined voltage level is applied to the electrodes to generate an electric field normal to the surfaces of the water-splitting membrane. The electric field irreversibly dissociates water to split it into its component ions H+ and OH− that migrate through the ion exchange layers in the direction of the electrode having an opposite polarity (e.g., H+ migrates toward the negative electrode). The electric field assists the transport of the dissolved solid ions generated by the water-splitting reaction in a direction perpendicular to the membrane to provide a short pathway through the membrane. Thus, during deionization, the electric field is set at a single level that is sufficiently high to dissociate the water and effectively transport and remove a large majority of dissolved solids from the solution. The predetermined fixed field strength is generated by applying a single high DC voltage to the electrodes that maximizes ion extraction. Electrolytic ion exchange advantageously provides a uniform electric field in the cell that better utilizes the entire surface area of the membrane and that increases ion exchange efficiency to be able to remove 90% or more of the dissolved solids.
Electrolytic assisted ion exchange systems also allow electrical regeneration of the membranes, which is advantageous over conventional chemical regeneration processes. Conventional cation exchange layers are commonly regenerated using acidic solutions, such as sulfuric acid; and anion exchange layers are regenerated using basic solutions, such as sodium hydroxide. Regeneration is concluded with a rinsing step that removes entrapped regenerant solution. These chemical processes require large amounts of regenerating chemicals and/or water, and the cell has to be periodically shut down to allow the regeneration process. However, in electrolytic assisted ion exchange processes, the water-splitting membrane is regenerated by simply reversing the polarity of the voltage applied to electrodes to generate an inverted electric field that electrically regenerates the membranes by disgorging exchanged ions from the membrane. The reverse polarity voltage is also applied at a single voltage level that maximizes ion expulsion and/or rejection efficiency from the water splitting membrane during the regeneration cycle.
However, conventional electrolytic assisted ion exchange systems have several limitations. One limitation is that the TDS removal fraction can vary with influent solution quality and cell operating conditions. For example, a change in the flow rate or pressure of the influent solution can result in different fractions of dissolved solids being removed. The total dissolved solids content of the effluent solution also varies due to the TDS content of the influent solution changing with time, for example, the TDS of sewage changes drastically with heavy rains. As a result, an electrolytic cell that provides a 90% TDS reduction will generate a treated effluent solution having a TDS of 15 ppm from an influent solution having a TDS of 150 ppm, but when the influent solution contains 1500 ppm TDS, the treated effluent solution will have a TDS of 150 ppm. Such variations in the output TDS content are undesirable. Conventional electrolytic ion exchange cells also often exhibit a gradual increase in TDS content in the effluent solution, as the working ion exchange capacity of the membrane is consumed during deionization, which further increases the variability in effluent solution TDS content. The slow increase of ion concentration during batch deionization processes can cause the premature end of the deionization cycle, well before the capacity of the membrane is truly exhausted, increasing capital and operating costs for the cell. Thus, the output TDS content can vary significantly with influent TDS content and over time with conventional systems.
Furthermore, conventional electrolytic ion exchange systems also do not allow control of the ion concentration. While maximizing extraction of ions from a solution stream is desirable to purify water and de-ionized water in industrial applications, in some applications, it is desirable to maintain a predefined level of dissolved solids in the solution stream. For example, in drinking and cooking water applications, some dissolved solids are desirable so the water tastes better and does not taste synthetic. In other industrial water applications, it is also desirable to reduce the level of a particular ion in a wastewater stream, for example nitrate or arsenic ions, to meet an environmental standard. A particular ion level in a chemical solution can also be needed to provide precise control of the composition of the solution for industrial processes, for example, in cement manufacture and in electroplating solutions.
Thus, it is desirable to be able to treat influent solutions to provide uniform and consistent ion concentrations in the effluent solution, even if the quality or TDS of the influent solution varies over the treatment process. There is also a need for a water treatment system that can use electrical power for regeneration of ion exchange materials that uses electrical power rather than chemicals for regeneration to reduce or eliminate the inconvenience and environmental hazards associated with regenerant chemicals, and reduce rinse water volumes during cleaning cycles. It is also desirable to be able to treat influent solutions to maintain a predetermined or set level of dissolved solids in the treated solution.